(1) Field
This invention relates to measurement systems using resistive sensors, which are sensors whose resistance varies according to the physical quantity that they measure.
It particularly applies to the field of imagers, where resistive sensors are bolometers, such as infrared bolometers whose resistance varies according to the infrared radiation received, or bolometers that are sensitive to other frequencies of electromagnetic radiation, such as terahertz frequencies.
In the remainder of the description and in the claims, the equality of two electrical values (voltage, current, resistance, etc.) means that these two values are equal with only a few technological dispersions.
Moreover, terms like “high”, “low”, “row”, and “column” will be used in reference to the orientation of the drawings for clarity in describing items. They are not intended to be limited specifically to the actual orientation and geometric layout of these elements.
(2) Description of the Related Art
An infrared imager provides an image in the form of a pixel array. In uncooled infrared imaging, bolometers are used to capture the infrared flux coming from a scene. There is usually one bolometer per pixel in order to produce an infrared image of the scene, each bolometer being integrated into a respective bolometric cell. Therefore, the infrared imager comprises an array of bolometric cells.
A bolometer is a resistive sensor whose resistance varies with the temperature and therefore with the infrared flux coming from the scene. To read the resistance value of the bolometer, which corresponds to an infrared flux, it is known to apply a voltage and to measure the current passing through the bolometer.
However, the variation in current related to a variation in infrared flux of 50 Kelvin is around a percent. It is therefore necessary to remove most of the measured current in order to have a current to integrate that is as small as possible corresponding to the resistance variations of the sensitive bolometer in response to the infrared flux from the scene. This removal of current is called “baselining”.
For this, it is known to use reference bolometers that are subjected to little or none of the effects of the infrared flux coming from the scene. These reference bolometers are either “column footing”, meaning that there is one reference bolometer common to all of the bolometers in a column, or “row heading”, meaning that there are one or more reference bolometers common to all of the bolometers in a row.
Thus, with reference to FIG. 1, in this second case, it is known to use an infrared imager comprising:                a resistive sensor such as a reference bolometer 2,        means 4, 6 for applying a voltage to the reference bolometer 2, such that the reference bolometer is traversed by a reference current,        a reference arm 8 with a reference resistance 10 and connected such that it is traversed by the reference current to produce a reference voltage between its ends,        at least one resistive sensor, such as a measurement bolometer 12, and for each measurement bolometer 12:                    means 4, 16 for applying a voltage to the measurement bolometer, such that the measurement bolometer is traversed by a measurement current that depends on the exposure of the measurement bolometer to infrared radiation,            a mirror measurement arm 18 with a resistance 20 and intended to be traversed by a current that is equal to the reference current,            means 22 for measuring the difference in current between the measurement current and the current traversing the mirror measurement arm 18.                        
In such an infrared imager, the means for applying a voltage to the reference bolometers and measurement bolometers comprise, for each bolometer, on the one hand, a power supply line 4 that polarizes a bolometer terminal and, on the other hand, a transistor 6 whose power source polarizes the other bolometer terminal.
Moreover, transistors 24, 26 in a mirror current arrangement are used to replicate the reference current in each of the mirror measurement arms of the given row in the array.
In such an arrangement, a first transistor 24 is diode-connected, which involves that it adjusts its gate potential to allow the arriving current that arrives to its drain to flow. In each measurement arm, the gate of a second transistor 26, which is not diode-connected, is connected to the gate of the first transistor 24 such that said second transistor 26 copies the current traversing the first transistor 24 between its source and its drain.
This solution provides two advantages, namely a significant rejection of power supply noise and low temperature drift, given that the reference circuit and the measurement circuits are very similar in terms of the components they use.
However, the current trend is to lower power supply voltages, which requires also reducing the values of bolometric resistances in order to maintain acceptable levels of performance.
However, using such bolometers leads to an increase in the reference current. This results in a higher gate potential of the transistor 24 and therefore also of the drain of the transistor 6 for polarizing the reference bolometer.
However, this polarization transistor 6 must operate in saturated mode, which is not possible if the drain potential is too high.
Therefore, the known infrared imager is incompatible with the current trend of lowering power supply voltages.
Moreover, the reduction in the values of bolometric resistances significantly limits the readout electronics such that, in order to keep the noise from this readout electronics insignificant relative to the bolometric cells, it becomes necessary to use large transistors with a high capacitance between their gate and their substrate. This is added to the fact that array formats always tend to grow larger such that the combined capacitance of the measurement transistors becomes very high and difficult to charge in a time that is compatible with the constraints of video rate.
Therefore, the known infrared imager is incompatible with the current trend of lowering power supply voltages and increasing the size of the array of bolometric cells.
It may therefore be desirable to provide a measurement system that can overcome at least some of the above problems and constraints.